CVE-2026-18446 (CVSS 7.5)

Description

fast-uri before 4.1.2, 3.1.5, and 2.4.4 requires a literal double forward slash to recognize a URI authority, so a reference that uses a backslash based introducer in place of it (backslash backslash, forward slash backslash, or backslash forward slash) is parsed with no authority and folds into the path. Node's native WHATWG URL parser instead treats a backslash as interchangeable with a forward slash for special schemes, so the two parsers extract different hosts from the same input. Applications that use fast-uri to enforce host based policy such as allowlists, SSRF filtering, or redirect validation before passing the same URL into Node's URL or fetch consumers can be steered to an unintended host. Upgrade to fast-uri 4.1.2, 3.1.5, or 2.4.4.

Is your site exposed to CVE-2026-18446?

Run a free security scan — no signup, results in seconds.

CVSS v3.1 Score

7.5

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS — Exploit Prediction

0.0022

Probability of exploitation

0.13%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-436 CWE-436

References

Other References

https://cna.openjsf.org/security-advisories.html https://github.com/fastify/fast-uri/security/advisories/GHSA-7p8r-x3mc-p8w7

Frequently Asked Questions

What is CVE-2026-18446? +

fast-uri before 4.1.2, 3.1.5, and 2.4.4 requires a literal double forward slash to recognize a URI authority, so a reference that uses a backslash based introducer in place of it (backslash backslash, forward slash backslash, or backslash forward slash) is parsed with no authority and folds into the path. Node's native WHATWG URL parser instead treats a backslash as interchangeable with a forward slash for special schemes, so the two parsers extract different hosts from the same input. Applications that use fast-uri to enforce host based policy such as allowlists, SSRF filtering, or redirect validation before passing the same URL into Node's URL or fetch consumers can be steered to an unintended host. Upgrade to fast-uri 4.1.2, 3.1.5, or 2.4.4. It has a CVSS v3.1 base score of 7.5 (HIGH).

How severe is CVE-2026-18446? +

CVE-2026-18446 has a CVSS v3.1 score of 7.5 out of 10, rated HIGH. This is a high-severity vulnerability that should be prioritized for patching. The EPSS score is 0.0022, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-18446? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

Related Vulnerabilities

CVE-2026-116265

MIME::Field::ParamVal versions before 5.521 for Perl do not flag an ambiguous multipart boundary written as RFC 2231 continuations. The boundary …

CVE-2026-116269

MIME::Field::ParamVal versions before 5.521 for Perl turn an RFC 2231 filename with a hyphenated language tag into an empty recommended …

CVE-2026-84747

Interpretation conflict in the HTTP protocol implementation in Apache bRPC (version < 1.18.0) on all platforms allows remote attackers to …

CVE-2026-116266

MIME::Head versions from 5.518 before 5.521 for Perl decode a partly encoded multipart boundary without flagging the parse as ambiguous. …

CVE-2026-116695

Crass is a Ruby CSS parser that is compliant with the CSS Syntax Level 3 specification. Prior to 1.0.8, Crass …

CVE-2026-42272

Heimdall is a cloud native Identity Aware Proxy and Access Control Decision service. Prior to version 0.17.14, Heimdall handles URL-encoded …

CVE-2026-18446